Raman scattering is a type of inelastic scattering of electromagnetic radiation, such as visible light, discovered in 1928 by Chandrasekhara Raman. When a beam of monochromatic light is passed through a substance some of the radiation will be scattered. Although most of the scattered radiation will be the same as the incident frequency (“Rayleigh” scattering), some will have frequencies above (“anti-Stokes” radiation) and below (“Stokes” radiation) that of the incident beam. This effect is known as Raman scattering and is due to inelastic collisions between photons and molecules that lead to changes in the vibrational and/or rotational energy levels of the molecules. This effect is used in Raman spectroscopy for identifying and investigating the vibrational and rotational energy levels of molecules. Raman spectroscopy is the spectrophotometric detection of the inelastically scattered light.
“Stokes” emissions have lower energies (lower frequencies or a decrease in wave number (cm−1)) than the incident laser photons and occur when a molecule absorbs incident laser energy and relaxes into an excited rotational and/or vibrational state. Each molecular species will generate a set of characteristic Stokes lines that are displaced from the excitation frequency (Raman shifted) whose intensities are linearly proportional to the density of the species in the sample.
“Anti-Stokes” emissions have higher frequencies than the incident laser photons and occur only when the photon encounters a molecule that, for instance, is initially in a vibrational excited state due to elevated sample temperature. When the final molecular state has lower energy than the initial state, the scattered photon has the energy of the incident photon plus the difference in energy between the molecule's original and final states. Like Stokes emissions, anti-Stokes emissions provide a quantitative fingerprint for the molecule involved in the scattering process. This part of the spectrum is seldom used for analytical purposes since the spectral features are weaker. However, the ratio of the Stokes to the anti-Stokes scattering can be used to determine the sample temperature when it is in thermal equilibrium.
The Stokes and anti-Stokes emissions are collectively referred to as spontaneous Raman emissions. Since the excitation frequency and the frequency of the Stokes (and anti-Stokes) scattered light are typically far off the excitation of any other component in the sample, fluorescence in near infrared (NIR) wavelengths is minimal. The sample is optically thin and will not alter the intensities of the Stokes emissions (no primary or secondary extinctions), in stark contrast to infrared spectroscopy.
Raman spectroscopy is a well-established technology to determine the presence of trace compounds down to very low (e.g. n mol/liter) levels. With Raman analysis, absolute densities can be determined, the sparse spectra minimize interferences, and overtones and combination lines are strongly suppressed.
However, conventional Raman analyzers tend to lack the desired sensitivity, require an extensive integration time, be too large, and/or be too costly for widespread use. Thus, there is a need in the art for a relatively inexpensive, compact Raman spectrometer capable of improved sensitivity and integration times.
Laser-based techniques capable of detecting very small traces of inorganic compounds have been recently reported in the literature. However, these instruments generally require the use of tunable lasers and special environments like a vibration-free setting. Often the experimental setups are so sophisticated that they can be operated only by Ph.D. level personnel.
A novel approach is presented here. Raman spectroscopy is often used for identification and quantization of a mixture of chemical species with high selectivity. In a typical Raman experiment, a laser is used as an excitation source. Scattered light is collected and sent to a grating spectrograph connected to a detector, typically a charge-coupled device (CCD). Elastically scattered (Rayleigh) light is rejected by a narrow atomic vapor filter.
There are many Raman systems on the market today; however, they tend to suffer from the same drawbacks. Raman cross sections are extremely small; therefore, only dense materials (solids or liquids) in sufficiently large quantities can be routinely detected by these instruments. Raman spectrometers capable of detecting low densities of gaseous substances have been reported in elaborate intra-cavity laser setups, but these techniques require sophisticated frequency stabilization and can be achieved today only in state-of-the-art laboratories, without much hope for deployment in the field.